mibuild: platforms: adding apf27

Hi all,

Here is a platform description for armadeus board apf27. This board has a FPGA Spartan3A from Xilinx.

I managed to synthesize the following blinking led tutorial : 
```
from migen.fhdl.std import *
from mibuild.platforms import apf27

plat = apf27.Platform()
led = plat.request("IO_L24P_1")  # led pin on apf27dev
m = Module()
counter = Signal(26)
m.comb += led.eq(counter[25])
m.sync += counter.eq(counter + 1)
plat.build_cmdline(m)
```
But I had a XST bug in bitstream generation : 
```
ERROR:Xst:2883 - Option "-reduce_control_sets" is not available for the selected device family.
```
(with ISE 14.7 webpack)